1. Field of the Invention
The present invention relates to a device and an optical element for the aiming and the visual indication of a reading area of a coded information reader. More in particular, the invention relates to a device and an optical element for aiming a reading area of a coded information reader (preferably, but not exclusively, a portable reader) for providing the operator with a visual indication of the reading area framed by the reader so as to optimise the subsequent reading operations.
The invention also relates to a coded information reader comprising an aiming device of the type described above.
2. Related Art and Other Considerations
As known, in recent years coded information readers (for example optical, magnetic and radiofrequency readers) have been proposed on the market, which are capable of localising and decoding the information contained on a support (for example, an optical, magnetic or electronic code associated to an object) placed into a predetermined reading area so as to acquire the information.
In this description and following claims, the expression “coded information reader” indicates any device capable of acquiring information relating to an object (for example distance, volume, overall dimensions, or its identification data) through the acquisition and the processing of a luminous, magnetic or radio frequency signal diffused by the same object. The expression “coded information” indicates optical, magnetic and electronic codes. The expression “optical code” indicates any graphical representation having the function of storing a coded information. A particular example of optical code comprises linear or two-dimensional codes, wherein the information is coded through suitable combinations of dark-coloured elements (usually black) having a predetermined shape, for example square, rectangular or hexagonal, separated by clear elements (spaces, usually white), such as bar codes, stacked codes, and two-dimensional codes in general, color codes, etc. Moreover, the expression “optical code” also comprises, more in general, other graphical shapes having the function of coding the information, including characters printed in clear (letters, numbers, etc.) and particular patterns (such as for example, stamps, logotypes, signatures, fingerprints, etc.). The expression “optical code” also comprises graphical representations that are detectable not only in the field of visible light, but also in the wave band comprised between infrared and ultraviolet.
Typically, coded information readers comprise static or dynamic means for illuminating the reading area containing the code to be read with a luminous, magnetic or radio frequency flux, means for collecting the luminous, magnetic or radio frequency flux emitted by said area, means for converting said luminous, magnetic or radio frequency flux into an analogue or digital electric signal, and means for processing said electric signal so as to extract the information contained in the code.
For the purpose of facilitating the arrangement of the reader with respect to the reading area, thus guaranteeing a correct reading of the information contained therein, it is very important to make the operator aware of the position and the extension of the area framed by the reader. For this purpose, devices for the aiming and/or the visual indication of the reading area framed by optical readers have been developed. Such devices are typically adapted to be mounted into the optical readers in a more or less misaligned position with respect to the reader optical axis.
Aiming optical devices are known which are capable of providing the operator with a visual indication of the framed area through the identification of the center and/or the edges, or of the outline, or of a combination of them, of the area framed by the reader.
For example, European patent application no. 98830656.9 by the same Applicant describes an optical device comprising a plurality of illumination groups, each one including a luminous source, a diaphragm having a predetermined profile, and a converging lens arranged, in the optical emission path, downstream of the diaphragm and adapted to collimate the light beam coming from the diaphragm and project it onto an end portion of the reading area. This device provides for the use of more luminous sources (typically, LED or laser sources), each one adapted to illuminate a corresponding end portion of the reading area. Nevertheless, the use of more sources unavoidably increases the size and cost of the aiming device. Size and cost increase are often undesirable, particularly in portable optical readers.
The U.S. Pat. No. 5,500,702 describes an optical device comprising an holographic (HOE) or diffractive (DOE) optical element arranged downstream of a laser source and of a collimation lens, and adapted to deflect the collected light beam so as to project a plurality of different light beams onto different end portions of the reading area. The device of U.S. Pat. No. 5,500,702 uses a single luminous source for illuminating more end portions of the reading area.
A first drawback associated to a device of the type described above relates to the high cost and to the construction difficulty of the holographic and/or diffractive optical elements used. These drawbacks are essentially due to the difficulty of realizing, on their surface, grooves of minimum sizes. Such sizes depend on the divergence to be imparted to the various light beams, and they decrease as the beam divergence increases.
In fact, it is desirable that the area framed by the aiming device corresponds to the area framed by the reader at all reading distances. Particularly for relatively wide reading ranges (for example, wider than 30°), it is possible to obtain such a divergence with a diffractive (DOE) or holographic (HOE) element by reducing the minimum sizes of the grooves up to about 1 μm or less for greater view angles, as it results from the calculation example reported below.
In fact, in a first approximation, it is possible to calculate the minimum size of the grooves on the surface of an HOE or DOE with the same design rules as used for diffraction gratings. With θ representing half the field of view angle of the reader, λ the wavelength of the incident radiation on the DOE (HOE) and d the minimum size of the groove, the expression for λ is given by the following relation:λ=2d·sin θFrom this relation it results that, for a field of view of 2θequal to 30° and an incident radiation λ equal to 650 nm, the minimum size d of the groove must be equal to 1.26 μm, that is to say, very small.
A second drawback associated with a device of the type described above relates to the fact that, for using a DOE (HOE), it is necessary to have a coherent source (laser source). As is known, coherent sources are particularly expensive.
The technical problem at the basis of the present invention is that of providing an aiming device which should be, at the same time, economic and easily constructed, yet also for sufficiently high divergence angles, and capable of providing the operator with a clear and precise indication of the framed reading area, independently of the distance at which the latter is with respect to the same device.